Red blood cells and other blood cell types develop from a relatively undifferentiated population of cells called stem cells. Under the influence of different microenvironments in hemopoietic organs, stem cells become committed to restricted pathways of differentiation. Stem cells thus committed are said to be determined progenitor cells. Since it is probable that a number of diseases of blood cells, including certain leukemias and anemias, are due to alterations in stem cells and/or progenitor cells or due to deficiencies in the hemopoietic microenvironments which influence the differentiation of these precursor cells, further study of the early stages of blood cell differentiation is important. Differentiation of red blood cells from stem cells results in expression of genes coding for molecules of hemoglobin, the oxygen-binding protein of red cells; the presence of hemoglobin in a cell is evidence that the cell has differentiated along the erythroid pathway. This proposal is concerned with the differentiation of red blood cells in the bullfrog Rana catesbeiana. Bullfrog tadpoles have multiple hemoglobins which represent products of different genes. I have shown that two erythropoietic organs synthesize different tadpole hemoglobins in organ cultures, indicating that red blood cells containing different hemoglobins differentiate in the two organs. The main goal of this research is to investigate specific factors of the two organ environments that are involved in mediating this difference in gene expession during red blood cell differentiation. It is hypothesized that the microenvirnments of erythropoietic organs are responsible for restricting or otherwise programming specific gene expression that becomes evident in later stages of red blood cell differentiation. Organ cultures, cell cultures and cultures of reaggregated, previously separated cell populations from erythropoietic organs and whole blood will be used to delineate the early cellular and molecular events in the differentiation of red blood cells from stem cells and or progenitor cells.